Draw a meteogram, and let the output go somewhereHEADv0.1.4main

-svg renders the forecast as a standalone SVG: temperature with the apparent
temperature dashed, precipitation as bars with the probability on its own
right-hand axis, and humidity, over shaded night bands with a rule at each
midnight. The rain panel carries two units, so the probability gets a second
axis; read against millimetres a 60% chance looks like 0.6 mm.

Written as markup rather than through gnuplot or a plotting library, so it
needs nothing installed and works wherever the binary does -- the phone
included. Axis ticks are snapped to round values and the number of gridlines
follows the step, since snapping only the ends still yields 12.5 and 27.5.
An hour label that would land under a bold date is dropped, or "23" and
"28.08" render on top of each other.

-o and -out choose the destination, for the table as much as the meteogram. A
directory gets a filename made from the place and the date, so a daily run does
not overwrite yesterday; a bare name gains the extension, so nothing lands as a
file no viewer will open. Colour is omitted when writing to a file. The place
is transliterated rather than stripped, so Zbylitowska Gora keeps its vowels.

Documented which viewers accept a pipe: feh - and chafa do, nsxiv does not and
answers with its usage, which hides the cause. The earlier claim that
ImageMagick renders these files wrong was too broad -- that is convert falling
back to its own renderer; display delegates to rsvg-convert and is fine.

1 files changed, 215 insertions, 0 deletions

diff --git a/internal/render/svg_test.go b/internal/render/svg_test.go
new file mode 100644
index 0000000..075cb72
--- /dev/null
+++ b/internal/render/svg_test.go
@@ -0,0 +1,215 @@
+package render
+
+import (
+	"encoding/xml"
+	"math"
+	"regexp"
+	"strconv"
+	"strings"
+	"testing"
+	"time"
+
+	"github.com/lukaszkasprzak/prognosis/internal/openmeteo"
+)
+
+// svgView builds hours starting at 12:00, so a 24-hour view crosses midnight.
+func svgView(hours int) View {
+	v := View{Label: "Krakow, PL", Sun: map[string][2]string{}}
+	start := time.Date(2026, 8, 27, 12, 0, 0, 0, time.UTC)
+	for i := 0; i < hours; i++ {
+		when := start.Add(time.Duration(i) * time.Hour)
+		v.Rows = append(v.Rows, openmeteo.Row{
+			When: when,
+			Code: 3,
+			Vals: map[string]float64{
+				"temperature_2m":            15 + 6*math.Sin(float64(i)/4),
+				"apparent_temperature":      14 + 6*math.Sin(float64(i)/4),
+				"precipitation":             float64(i%7) * 0.1,
+				"precipitation_probability": float64(i % 100),
+				"relative_humidity_2m":      50 + float64(i%40),
+			},
+		})
+		v.Sun[when.Format("2006-01-02")] = [2]string{"05:47", "19:35"}
+	}
+	return v
+}
+
+func svgOf(t *testing.T, hours int) string {
+	t.Helper()
+	out := SVG(svgView(hours), testConfig())
+	if out == "" {
+		t.Fatal("SVG produced nothing")
+	}
+	return out
+}
+
+func TestSVGIsWellFormedXML(t *testing.T) {
+	out := svgOf(t, 24)
+	if err := xml.Unmarshal([]byte(out), new(any)); err != nil {
+		t.Fatalf("not well-formed XML: %v", err)
+	}
+	if !strings.HasPrefix(out, "<svg xmlns=") {
+		t.Error("missing the SVG namespace, so viewers will refuse it")
+	}
+}
+
+// A place name is user input and can carry characters XML reserves.
+func TestSVGEscapesTheLabel(t *testing.T) {
+	v := svgView(6)
+	v.Label = `Foo & <Bar>`
+	out := SVG(v, testConfig())
+	if strings.Contains(out, "<Bar>") || strings.Contains(out, "& ") {
+		t.Errorf("label was not escaped:\n%s", out)
+	}
+	if err := xml.Unmarshal([]byte(out), new(any)); err != nil {
+		t.Fatalf("unescaped label broke the XML: %v", err)
+	}
+}
+
+func TestSVGEmptyViewProducesNothing(t *testing.T) {
+	if got := SVG(View{}, testConfig()); got != "" {
+		t.Fatalf("expected empty output for no rows, got %d bytes", len(got))
+	}
+}
+
+// One point per hour, on every series.
+func TestSVGSeriesHaveAPointPerHour(t *testing.T) {
+	const hours = 18
+	out := svgOf(t, hours)
+	polylines := regexp.MustCompile(`<polyline points="([^"]+)"`).FindAllStringSubmatch(out, -1)
+	if len(polylines) < 4 {
+		t.Fatalf("expected at least four series, found %d", len(polylines))
+	}
+	for i, m := range polylines {
+		if n := len(strings.Fields(m[1])); n != hours {
+			t.Errorf("series %d has %d points, want %d", i, n, hours)
+		}
+	}
+}
+
+// Night shading is what makes a meteogram readable; without sun data there must
+// be none rather than a guess.
+func TestSVGNightBandsFollowSunData(t *testing.T) {
+	withSun := svgOf(t, 24)
+	if !strings.Contains(withSun, `fill="#eef1f6"`) {
+		t.Error("no night bands drawn despite sunrise/sunset being known")
+	}
+	v := svgView(24)
+	v.Sun = map[string][2]string{}
+	if strings.Contains(SVG(v, testConfig()), `fill="#eef1f6"`) {
+		t.Error("night bands drawn without sun data; that is a guess")
+	}
+}
+
+func TestIsNight(t *testing.T) {
+	sun := map[string][2]string{"2026-08-27": {"05:47", "19:35"}}
+	at := func(h int) time.Time { return time.Date(2026, 8, 27, h, 0, 0, 0, time.UTC) }
+	for _, c := range []struct {
+		hour int
+		want bool
+	}{{0, true}, {5, true}, {6, false}, {12, false}, {19, false}, {20, true}, {23, true}} {
+		if got := isNight(at(c.hour), sun); got != c.want {
+			t.Errorf("%02d:00 night=%v, want %v", c.hour, got, c.want)
+		}
+	}
+	// An unknown date must not be shaded on a guess.
+	if isNight(time.Date(2030, 1, 1, 3, 0, 0, 0, time.UTC), sun) {
+		t.Error("shaded a date with no sun data")
+	}
+}
+
+// The rain panel carries millimetres and percent. Without the right-hand axis
+// the dashed probability line is read against the millimetre scale.
+func TestSVGRainPanelHasBothAxes(t *testing.T) {
+	out := svgOf(t, 24)
+	if !strings.Contains(out, "100%") || !strings.Contains(out, "50%") {
+		t.Errorf("no percentage axis for the probability line:\n%s", out)
+	}
+}
+
+func TestSVGDatesAreBoldAndHoursDoNotCollide(t *testing.T) {
+	out := svgOf(t, 30) // crosses midnight
+	if !strings.Contains(out, `font-weight="bold"`) {
+		t.Error("date labels are not bold")
+	}
+
+	// Collect the x of every axis label, and check none sits on top of a date.
+	type label struct {
+		x    float64
+		bold bool
+	}
+	var labels []label
+	re := regexp.MustCompile(`<text x="([0-9.]+)" y="\d+" text-anchor="middle"( font-weight="bold")?`)
+	for _, m := range re.FindAllStringSubmatch(out, -1) {
+		x, _ := strconv.ParseFloat(m[1], 64)
+		labels = append(labels, label{x: x, bold: m[2] != ""})
+	}
+	if len(labels) < 4 {
+		t.Fatalf("expected several axis labels, found %d", len(labels))
+	}
+	for _, a := range labels {
+		if !a.bold {
+			continue
+		}
+		for _, b := range labels {
+			if !b.bold && math.Abs(a.x-b.x) < 20 {
+				t.Errorf("an hour label at x=%.0f collides with the date at x=%.0f", b.x, a.x)
+			}
+		}
+	}
+}
+
+func TestSVGHasALegendForTheTwoTemperatureSeries(t *testing.T) {
+	out := svgOf(t, 12)
+	if strings.Count(out, ">temp<") == 0 || strings.Count(out, ">feels<") == 0 {
+		t.Errorf("the dashed series is unexplained without a legend:\n%s", out)
+	}
+}
+
+func TestSVGDrawsDayBoundaries(t *testing.T) {
+	crossing := svgOf(t, 30)
+	if !strings.Contains(crossing, `stroke="#9aa3ad"`) {
+		t.Error("no midnight rule drawn on a view that crosses midnight")
+	}
+	if strings.Contains(svgOf(t, 6), `stroke="#9aa3ad"`) {
+		t.Error("midnight rule drawn on a six-hour view that never reaches midnight")
+	}
+}
+
+// Axis labels must land on round numbers, and the count must follow the step.
+func TestNiceTicks(t *testing.T) {
+	for _, c := range []struct{ lo, hi float64 }{{9.4, 22.6}, {0, 1}, {-4.2, 3.1}, {990, 1030}} {
+		lo, hi, lines := niceTicks(c.lo, c.hi, 6)
+		if lo > c.lo || hi < c.hi {
+			t.Errorf("niceTicks(%v,%v) = %v,%v — must contain the range", c.lo, c.hi, lo, hi)
+		}
+		if lines < 2 || lines > 8 {
+			t.Errorf("niceTicks(%v,%v) wants %d lines", c.lo, c.hi, lines)
+		}
+		step := (hi - lo) / float64(lines-1)
+		for i := 0; i < lines; i++ {
+			v := lo + step*float64(i)
+			if math.Abs(v-math.Round(v*100)/100) > 1e-9 {
+				t.Errorf("tick %v is not a round value", v)
+			}
+		}
+	}
+}
+
+// The meteogram needs its own fields whatever the table shows.
+func TestSVGFieldsCoverEveryPanel(t *testing.T) {
+	for _, f := range []string{
+		"temperature_2m", "apparent_temperature", "precipitation",
+		"precipitation_probability", "relative_humidity_2m",
+	} {
+		found := false
+		for _, have := range SVGFields {
+			if have == f {
+				found = true
+			}
+		}
+		if !found {
+			t.Errorf("SVGFields is missing %q, so that panel would be empty", f)
+		}
+	}
+}